Small fix for NTXentLoss with no negative pairs

[stephantul]

If all instances in a single batch have the same class, the NTXentLoss crashes with an uninformative warning:

AttributeError: 'list' object has no attribute 'dtype'

This is because this line assigns an empty list to neg_pairs if there are no negative pairs.
This crash can be avoided by moving the check for dtype in NTXentLoss (i.e., this line) a bit down, so it is enclosed by the if statement.

Should be a rare occurrence anyway, but I'd thought I'd signal it nonetheless 🐱

I can do a PR if you'd like, but maybe this is such a small fix that it doesn't need a separate PR

[KevinMusgrave]

Thanks for finding this bug. I've fixed it in the latest commit